Many medical treatment regimens require the monitoring of specific physiological signals. For example, the treatment of various heart problems often depends on the collection and subsequent analysis of electrocardiogram (ECG) signals from a patient. Many abnormal heart rhythms that would be diagnostic in the treatment process, however, may occur with intermittent frequency and with little predictability. These abnormal heart rhythms may thus be unavailable to a medical practitioner when the patient's heart rhythms are monitored for only limited periods of time. In order to allow the collection of these intermittent, abnormal rhythms, or to monitor long stretches of heart activity over time, a portable heart monitor may be utilized. Portable heart monitors allow patients to escape the confines of a hospital bed and to be able to go about normal life while still allowing the collection of heart rhythms for diagnostic purposes.
Heart rhythms are generally digitally sampled by an analogue to digital converter and stored in physical memory. Each point of the sampled signal is typically represented by two numeric values, one for amplitude and the other for time. A higher sampling rate requires more physical memory storage to store the signal. While decreasing the sampling rate will decrease the size of the recorded signal, unacceptable degradation to the reconstituted signal can occur from a diagnostic point of view. For example, some cardiac waveforms contain energy above 60 Hz, and thus a sampling rate below 60 Hz will result in the loss of that signal information.
While physical memory limitations tend to be insignificant in hospital and clinical locations, portable heart monitors have limited memory storage. These memory limitations are even more severe in implantable heart monitors where memory storage space, power limitations, processing time, and cost are prime considerations.
Traditional data compression techniques may alleviate this problem to some extent but prove to be inflexible to the often dynamic needs of the medical practitioner and the patient. For example, if the cardiac signal is highly compressed to allow for a maximum recording time, but the patient wears the heart monitor for a much shorter length of time, signal information can be needlessly lost that may be vital to the diagnostic process. Conversely, if the cardiac signal is lightly compressed to allow for the recording of maximum signal information, and the patient wears the heart monitor for a longer period of time than the memory storage allows, signal data can be lost that may be vital to the diagnostic process.